Saturn is the jewel of the solar system. The giant, pastel-colored planet is surrounded by huge, iconic rings – and dozens of sparkling, icy moons.
Ten years ago today, NASA’s Cassini spacecraft gently pulled into orbit around the ringed planet. The journey to Saturn took an arduous seven years and covered 3.5 billion kilometers. But the trek was essentially a warm-up for what would become a tireless, gymnastic exploration of the Saturnian system. (I’m in Washington, D.C. today for a National Geographic live event commemorating Cassini’s accomplishments.)
For the last decade, Cassini has been continually looping through the system, flying high above the planet, swooping low over its satellites, and swiveling to stare at the next enigmatic target. It even dropped the European Space Agency’s Huygens lander onto the surface of Titan, Saturn’s largest moon. The spacecraft has collected hundreds of gigabytes of data and snapped thousands of photos (a few of my favorites are in the gallery above), and beamed all of its observations back to eager scientists on Earth. Some of Cassini’s photos have even included Earth itself – visible as a pale blue smudge tucked into the spaces between the planet’s frigid rings.
As Cassini swung through the picturesque planetscape, dynamic — sometimes violent — stories began to emerge.
Cassini saw that the planet’s rings, made of small, icy particles, are constantly shifting. Ghostly spokes and braided patterns swim through parts of the disk. One of these patterns, spied in Saturn’s C-ring, is the twisted fingerprint of a comet that smashed into the rings around the time the Black Death was running around Europe (scientists figured this out by mathematically unwinding the twisty pattern – kind of the equivalent of a planetary ring rewind).
The rings hide more than relics of ancient impacts. Surfing through the particulate sea are tiny moonlets, perhaps hundreds of them, that are invisible except for the propeller-shaped wakes they create. Small shepherd moons, like potato-shaped Prometheus and Pandora, prune the rings, sometimes creating gaps. And there are strange, 2.5-kilometer tall features that cast shadows over the super-flat disk, which average about 30 meters thick.
And then there’s Saturn itself. When Cassini arrived in 2004, the planet’s northern hemisphere was emerging from the throes of winter, its pole a deep, otherworldly blue. Now, one-third of a Saturn year later, summer is arriving in the north. There, the planet’s jetstream churns away and imprints a curious hexagonal shape around the planet’s crown. Simultaneously, the planet’s poles have been lit by beautiful, strange auroras – the likes of which aren’t seen anywhere else in the solar system. Under Cassini’s watchful eye, the clouds crossing Saturn’s enormous face have morphed and swirled. In late 2010, an enormous storm began brewing; it started as a great white spot and eventually wrapped itself around the entire planet (complete with a massive amount of lightning).
But if the planet is remarkable, its moons, arguably, are more so. Many of these tiny worlds are so different from one another, and so bizarre, that scientists puzzle over how they could have formed — how did bright white, pockmarked Tethys grow from the same building blocks that produced hazy, orange, oily Titan? It’s the same kind of question astronomers ask when they consider how such different planets emerged from the same disk of material swirling around an infant sun…just on a smaller scale.
Some of the moons likely formed during giant, cataclysmic collisions early in the planet’s history. Others, like dark, far-flung Phoebe, are probably interlopers – objects from the outer-outer solar system that scientists suspect have been captured by Saturn’s gravity. Some moons offer few clues and many questions – like Iapetus, with its two-toned coloration, enormous landslides, and strange, equatorial mountain range. Rhea might have once had a ring (or not), Dione a tenuous oxygen atmosphere (and a massive series of canyons gouged through one hemisphere). Mimas, with its huge, carved out crater, improbably looks like the Death Star.
And, the biggest mystery of all: Some of the moons might not be dead, lifeless worlds, but places where alien microbes could be stirring and slicing up molecules to produce energy.
Now, thanks to data from Cassini, we know that two of Saturn’s moons are among the best places to look for extraterrestrial life in the solar system.
Tiny Enceladus spews warm, salty water from giant fractures in its southern hemisphere; Cassini observations suggest that all the ingredients needed to build life are right there, tucked beneath the moon’s icy shell and wafting into outer space. Enormous Titan, the largest of the moony contingent, clings to a dense atmosphere that might be older than Saturn itself, and has a surface very similar Earth’s – except that where there’s water on Earth, there are liquid hydrocarbons on Titan. If scientists find life on Titan’s windswept surface, it will be in a form that’s vastly different from what we know on Earth. It will be unmistakably alien.
These chapters of Cassini’s adventures in the Saturnian system have been an incredible odyssey to follow. And the good news is, the book isn’t completely written. If all goes well, Cassini could continue to twirl around the system for another three years. Then, when its fuel supply runs out in 2017, the spacecraft will plunge toward Saturn’s surface, desperately beaming data back to Earth until it crumples beneath the weight of the planet’s atmosphere.